1. Field of the Invention
The present invention relates to a bearing structure for a cam shaft in internal combustion engines and, more particularly, a bearing structure for a cam shaft which is driven by a timing belt.
2. Description of the Prior Art
In overhead cam engines, the cam shaft is conventionally mounted in the engine so that its bearing portion is laid in a semi-circular journal-receiving cavity in an upper face of the cylinder head. A bearing cap, having a second semi-circular journal-receiving cavity provided in a lower face thereof, is mounted with the second cavity opposing the first mentioned cavity in the cylinder head, thereby enabling the bearing portion of the cam shaft to be rotatable in the round bearing cavity defined by said first and second semi-circular cavities. The bearing surface of this structure is generally formed with an annular oil groove which is supplied with oil under a pressure such as 2 Kg/cm.sup.2 -4 Kg/cm.sup.2 in normal operation and 6 Kg/cm.sup.2 when starting-up at a low temperature. The oil is generally supplied through a passage which opens into the bearing surface provided by the cylinder head. When the cam shaft supported by this bearing structure is driven by gear wheels or an endless chain, the driving means are located within the body of the engine and no problem arises even if the lubricating oil leaks out either from the front or rear of the bearing portion because the oil is positively received and collected in an oil pan provided at a lower portion of the engine. However, in those engines where the cam shaft is driven by a timing belt, the cam shaft has a front end portion which projects forward from the body of the engine and supports a pulley on which is mounted a timing belt. The latter also runs around another pulley mounted on the crank shaft of the engine and, because of this, the oil can no longer be collected if it has leaked out forward from the bearing portion.
To meet this problem, the conventional bearing structure for a cam shaft driven by a timing belt generally incorporates oil seal means provided between the front end portion of the bearing surface and the journal portion of the cam shaft so as to check and collect any oil leaked out from the bearing portion and to return the oil to the internal space of a cylinder head through an oil return passage which connects the oil seal means and said internal space. However, the lubricating oil pumped to the bearing under pressure can enter the very small clearance left between the upper face of the cylinder head and the lower face of the bearing cap and ooze out from the space between the cylinder head and the bearing cap. It has been suggested to have the adjacent faces conact each other more accurately and closely in order to prevent such an oil leakage. However it is practically impossible to finish the joining surfaces to such high accuracy as to perfectly exclude any entry of the pressurised lubricating oil between adjacent faces. As an alternative measure to meet this problem, it has been suggested to provide an oil seal between the joining faces. However, this is difficult to accomplish in practice in view of the function and structure of the bearing portion. In addition, this would result in a higher engine cost.